Interleukin-7 (IL-7), is a 25 kDa cytokine of the hemopoietin family that plays important roles in lymphocyte differentiation, proliferation, and survival [1-4]. Human IL-7 cDNA encodes 177 amino acids (aa) that include a 25 aa signal peptide [3]. Human IL-7 shares approximately 60-63% aa sequence identity with mouse, rat, canine and feline IL-7, and 72-76% with equine, bovine, ovine, porcine, feline and canine IL-7. Human and mouse IL-7 exhibit cross-species activity [2, 3]. IL-7 protein is produced by a wide variety of cells in primary and secondary lymphoid tissues, including stromal epithelial cells of the thymus, bone marrow, and intestines [1, 2, 5]. Circulating IL-7 protein is limiting in healthy animals, but increases during lymphopenia [1, 6]. IL-7 signals through a complex of the IL-7 Receptor alpha subunit (IL-7 R alpha, also known as CD127) with the common gamma chain (gamma c) [1]. IL-7 contributes to the maintenance of all naive and memory T cells, mainly by promoting expression of the anti-apoptotic protein Bcl-2 [7-9]. It is required for optimal T cell-dendritic cell interaction [6]. IL-7 is expressed early in B cell development prior to the appearance of surface IgM [1, 5, 7]. In mouse, IL-7 activation of IL-7 R alpha is critical for both T cell and B cell lineage development, while in humans, it is required for T cell but not for B cell development [4, 7, 10, 11]. However, IL-7 functions in both mouse and human pro-B cells to suppress premature Ig light chain recombination during proliferative growth [12, 13].
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